Window sash counterbalances offset at least part of the weight of window sashes to make the sashes easier to lift and to hold the sashes stationary in various positions along a range of sash travel within a window frame. The window sash weight is offset by a lifting force that is maintained as uniform as possible throughout the range of sash travel to minimize opposite conditions of sash "hop" and sash "drop".
Too much lifting force causes the sashes to undesirably rise or "hop" from a position within the sash travel range. Too little lifting force allows the sashes to fall or "drop" from a position within the same range. However, friction within the sash counterbalances and between the sashes and frames compensates for some variation in the lifting force by providing a controlled resistance to any movement of the sashes within the frames.
Although some friction is desirable to compensate for variations in the lifting force, excessive friction can make the sashes difficult to move. Accordingly, both the friction and the variations in the lifting force are limited to obtain optimum overall performance of the sash counter-balances. For example, torsion spring balances can be used as window sash counterbalances to provide a nearly uniform amount of lifting force throughout the range of sash travel.
U.S Pat. No. 2,580,705 to Tappan discloses an example of a torsion spring balance including a torsion spring having one end secured against rotation and a follower nut attached to the other end for imparting angular movement between the two ends. A spiral rod having a varying pitch is threadably engaged with the follower nut for transforming a torque applied by the torsion spring into a lifting force against a window sash. The pitch of the spiral rod is varied to compensate for changes in the torque applied by the torsion spring.
U.S. Pat. No. 2,041,646 to Larson discloses an example of a tension spring combined with a torsion spring balance to provide additional lift capacity. The lifting force of the torsion spring balance is varied to compensate for variations in the lifting force of the tension spring throughout the range of sash travel. A spiral rod member of of the torsion spring varies in pitch to provide a decreasing amount of lift as the tension spring is extended.